Group inverses of $\{0,1\}$-triangular matrices and Fibonacci numbers

Abstract

A number $s$ is the sum of the entries of the inverse of an $n\times n,(n\ge 3)$ upper triangular matrix with entries from the set $\{0,1\}$ if and only if $s$ is an integer lying between $2-F_{n-1}$ and $2+F_{n-1}$, where $F_n$ is the $n$th Fibonacci number. A generalization of the sufficient condition above to singular, group invertible matrices is presented.